fix transaction id generation
fix transaction id generation

Incoming messages are matched to nodes in the retransmission
queue by comparing a hash value over (message id, port, ip addr).
For IPv4, the entire sockaddr_in structure was compared which
is potentially wrong (see merge request #7 from Dag Björklund).
Hopefully, this fixes the encountered issue.

gebart/libcoap,gebart/libcoap,thecodemaiden/libcoap-fork,authmillenon/libcoap,sgso/libcoap,authmillenon/libcoap,sgso/libcoap,fredericgermain/libcoap,gebart/libcoap,fredericgermain/libcoap,thecodemaiden/libcoap-fork,RIOT-OS/libcoap,nmacs/libcoap,nmacs/libcoap,fredericgermain/libcoap,sgso/libcoap,RIOT-OS/libcoap,authmillenon/libcoap,nmacs/libcoap